The present invention relates to the use of a preferably closed, extruded profile, to a development of a hollow profile, and to a process for producing a metal sheet which is stepped in cross-section and has different wall thicknesses, the profile or the hollow profile, for rolling the metal sheet, being stretched out flat before rolling.
U.S. Pat. No. 1,915,084 describes a process for producing a metal sheet which is stepped in cross-section and has different wall thicknesses. The prefabricated semifinished article has one or more flattenings running in the direction of rolling and is intended for producing the metal sheet. The semifinished article is designed such that it is already stepped at the points of the metal sheet which is to be rolled from the semifinished article and will later become steps, and likewise already has different wall thicknesses. The ratios of the wall thicknesses of appertaining regions of the semifinished article and of the finished metal sheet correspond to one another. That is, during rolling of the semifinished article to form the metal sheet, the wall thicknesses of the thicker regions of the semifinished article are reduced by approximately the same factor as the wall thicknesses of the thinner regions. For rolling the metal sheet, the semifinished article is laid in between two rolls. The roll associated with one side of the semifinished article or of the metal sheet has a planar rolling surface, while the roll associated with the other side of the semifinished article or of the metal sheet has a stepped rolling surface.
DE-OS 23 43 579 describes a further process and an apparatus for forming a metal sheet with a stepped cross-section and different material thicknesses. Pre-formed semifinished articles are used for producing the metal sheet by embossing rolling, die working, longitudinal welding on the borders of sheet-metal strips of different thicknesses, cutting, milling or another material-removing method of machining. However, the die working is expedient, however, only with semifinished articles which are comparatively short in the rolling direction, while the longitudinal welding, in particular in the case of light metals, and the material-removing method of machining cause extremely high outlays. In the same way as section rolling of the semifinished article to form the metal sheet, embossing rolling of a blank, designed as a metal block or slab, to form the semifinished article involves high outlay in terms of apparatus and energy. Thus, particularly with respect to the rolling of the semifinished article, the pressure which is to be applied and/or the energy requirement for tempering the metal of the blank during rolling can be very high. Furthermore, rolling trains of this type require a large amount of space, and control of the machine requires an extremely high outlay because, inter alia, the angular speed of those regions of the stepped roll which roll those portions of the metal sheet of a thinner wall thickness has to be lower than that of those regions of said roll which roll the thicker portions. These different angular speeds are therefore necessary in order that the tangential speed of these roll regions, having different diameters, is approximately the same.
DE-OS 14 52 646 describes a process for producing components, in particular closed hollow load-bearing members, whose wall thicknesses vary in accordance with the loading to which they are subjected. Furthermore, the hollow load-bearing member has only one wall, which turns in on itself in the peripheral direction without branching. A semifinished article is rolled from a sheet-metal strip, which is used as blank and is of a uniform cross-section.
The cross-sectional progression of said semifinished article is stepped and the different wall thicknesses of the semifinished article already corresponding to the desired wall thicknesses of the component which is to be formed therefrom. In this known arrangement, the cross-sectional progression of the semifinished article is selected such that the component can be bent from the semifinished article, with the regions of greater wall thickness being arranged at those points of the component which are subjected to higher loading. Finally, the free, abutting longitudinal borders of the semifinished article are then welded to one another.
An object of the present invention is to simplify the production of a metal sheet which is stepped in cross-section and has different wall thicknesses. It is a further object of the present invention for the metal sheet produced to exhibit, at least on one side, a surface finish which is suitable for an outer side, for example, of a vehicle body.
The foregoing objects have been achieved according to the present invention by the use of an extruded profile with different wall thicknesses in which the profile is stretched and flat before rolling, by a tubular hollow profile which is closed on the periphery, and by a process characterized in that for the production of the semifinished article, a hollow profile, having a wall-thickness progression distributed over the periphery thereof and corresponds to a desired wall-thickness progression of the semifinished article, is extruded from a blank, and the hollow profile is separated along a surface line and is formed into the semifinished article.
By using an extruded profile whose walls correspond to those of a semifinished article produced therefrom, at least a sub-process of the production of the semifinished article is simplified. In particular, this is favorable with metal sheets consisting of light metals because, inter alia, the extrusion method has been developed to an extremely advanced degree here. Furthermore, in comparison with iron or steel, the melting point of light metals is low, for which reason the amount of energy required in extrusion is also comparatively low. By rolling the semifinished article, a high surface finish of the metal sheet is achieved, and good dimensional stability of the metal sheet is also ensured simultaneously by rolling.